Subsurface safety valves have been used for many years in producing wells. These valves are generally operated by a movable sleeve. The movable sleeve holds the valve open in one position and allows a flapper element to close the passageway to the surface when placed in a second position. Typically, hydraulic control systems have been in use for actuation of the shifting tube to control the position of the subsurface safety valve. Generally, these hydraulic control systems involve a piston cylinder assembly which acts on the flow tube to open the safety valve. Some of these control systems have involved pressurized gaseous chambers which act on other movable pistons within the control system, and have been used in the past to facilitate the operation of the control system. Pressurized gas chambers counteract the hydrostatic pressure in the control line when the assembly is installed at depth. One of the problems that have occurred in such control systems involving pressurized gaseous chambers is that there is a precharge of pressure in the gaseous chamber which is precalculated for the given depth and installation of the subsurface safety valve. However, in the installation techniques, the control line sometimes needs to be taken apart prior to the subsurface safety valve having reached the appropriate depth. When those situations have arisen, there was a pressure imbalance because the hydrostatic head, before the predetermined depth was reached in the control line, was overcome by the precharged pressure in the gaseous chamber. Since installation techniques, particularly in subsea applications, required disconnection of a control line in order to facilitate the connection of a tubing hanger, the prior control systems, without the unique features as Will be discussed with regard to the present invention, posed the potential risk of having control fluid expelled from the control line at the time the disconnection was necessary.
Prior control systems also relied on a single valve actuated by control line pressure to open a fluid passage between the fluid in communication with a lower piston and the collection chamber, and further to close off communication between the lower piston and the upper piston. This type of a system had a disadvantage involving the time between the opening of the one fluid passage and the closure of the other. In an intermediate position, the control line pressure was in communication with all areas of the system. If the control line pressure and the flow rate were incapable of moving the valve quickly into its final position, the control line fluid would be pumped into the gaseous chamber.
Accordingly, a new control system has been developed to create a barrier between the gaseous chamber and other portions of the circuit so that the gaseous chamber pressure charge is not lost when the control line pressure is dropped, such as when the control line needs to be disconnected to connect a tubing hanger. The additional barrier piston which has been provided in the present invention overcomes the problem of the main piston adopting an intermediate position, which, in prior designs, allowed the fluid into the gaseous chamber. A boost piston also ensures full operation of the main piston if a system leak develops. Accordingly, another object of the apparatus and method of the present invention is to eliminate sensitivity by the control system to the rate at which pressure is applied to the system. In the event of leakages in critical areas, such as a gaseous leak or a hydraulic fluid leak, the actuating piston that operates the subsurface safety valve is placed in pressure balance so that the subsurface safety valve can close.